Why specifically are they wanting to change the hostname? If this has to do with exim, then the mailhelo can be changed for their IP if they have a dedicated IP by selecting WHM > Exim Configuration Editor for these options:

** Send HELO based on the domain name in /etc/mailhelo (*: HELONAME can be added to the file to change the default helo name) [?]

** Send outgoing mail from the IP that matches the domain name in /etc/mailips (*: IP can be added to the file to change the main outgoing interface) [?]

Click to expand...

After selecting these, then the dedicated IP entries can be manually added to each file:

/etc/mailips

Format:
resellerdomain.com: 192.0.0.0

/etc/mailhelo

Format:
resellerdomain.com: resellerdomain.com

/etc/mail_reverse_dns

Format:
192.0.0.0: resellerdomain.com

Above, replace resellerdomain.com with the name of the reseller's domain, then replace 192.0.0.0 with the dedicated IP for that reseller.

Of note, the PTR record in the IP zone will still be whatever the IP provider set for that IP. Whoever controls the IP DNS zone will need to change the PTR record for the IP to match the /etc/mail_reverse_dns file.